1. Field of the Invention
The present invention relates to a method of measuring temperature using a thermocouple, such as a ceramic thermocouple, both cold junctions, of which cannot be maintained at the same temperature, in cases where no compensating lead wires exist for the thermocouple or where, if any compensating lead wire for the thermocouple exists, it is difficult to manufacture for technical or economical reasons.
2. Description of the Prior Art
The relationship between the fundamental structure of a thermocouple and the thermoelectromotive force measured by the thermocouple is conceptually illustrated in FIG. 1. The thermocouple, indicated by X, comprises two dissimilar metals A and B joined at each end to form a hot junction P. The other ends of the metals A and B are maintained at 0.degree. C. or room temperature to form cold junctions q and r. In the illustrated example, these ends of the metals are kept at 0.degree. C. When the hot junction P is located on or in a material being tested, a thermoelectromotive force is developed between the cold junctions q and r. This force is measured to determine temperature.
The graph of FIG. 1 conceptually illustrates the thermo emf produced between the cold junctions q and r of the thermocouple X of the aforementioned structure. The thermoelectromotive power (in mV/.degree.C.), i.e. the rate of change with temperature of the thermo emf of the thermocouple, is plotted against the Centigrade scale. The temperature of the hot junction P is T.sub.0. The thermo emf is given by the product of the difference in thermoelectromotive power between the two wires of the thermocouple and the temperature. As an example, the thermo emf produced between the hot junction retained at T.sub.0 .degree. C. and the cold junctions maintained at 0.degree. C. corresponds to the area of the portion of the graph which is delineated by the thermoelectromotive characteristic lines of the two metals of the thermocouple within the temperature range from 0.degree. to T.sub.0 .degree. C. The thermoelectromotive characteristic is uniquely defined by the characteristic of the thermoelectric power. Therefore, if the two metals have the same thermoelectric power characteristic, then they have the same thermoelectromotive characteristic. The thermoelectric power characteristics (FIG. 1) of the two metals A and B were measured with calibrating thermocouples, each having a wire of platinum as its single metal, as shown in FIG. 2.
In FIG. 1, the thermo emf produced by the thermocouple X is denoted by the rectangular hatched region n.sub.1 n.sub.2 n.sub.4 n.sub.3 delineated by the thermoelectromotive power characteristic lines of the metals A and B, as mentioned previously.
FIG. 3 illustrates a method adopted in cases where the hot junction P is remote from a measuring instrument or where long metal wires coupled together to form a thermocouple are difficult to manufacture for technical or economical reasons. Compensating lead wires A' and B' are connected with the cold junctions q and r, respectively, of the thermocouple X. The wires A' and B' are made from dissimilar materials having thermoelectromotive characteristics agreeing with the thermoelectromotive characteristics of the two metals A and B at low temperatures. The thermo emf measured between the open ends q' and r' of the compensating lead wires A' and B', respectively, corresponds to the area of the portion n.sub.1 n.sub.2 n.sub.4 n.sub.3. In this example, the temperature of the hot junction P of the thermocouple X is T.sub.0. The cold junctions q and r are at the same temperature of T.sub.1. Therefore, the thermo emf directly generated by the thermocouple X corresponds to the area of the portion which is delineated by the thermoelectromotive characteristic lines of the two metals A and B in the temperature range from T.sub.0 to T.sub.1, i.e. the portion n.sub.5 n.sub.2 n.sub.4 n.sub.6. The thermo emf corresponding to the remaining portion n.sub.1 n.sub.5 n.sub.6 n.sub.3 is compensated by the compensating lead wires A' and B'. Since the thermoelectromotive power characteristics of the wires A' and B' agree with the thermoelectromotive power characteristics of the two metals A and B at low temperatures, the connection of the lead wires A' and B' with the metals A and B produces no further thermo emf. Hence, the temperature of the hot junction can be measured precisely.
As shown in FIG. 4, compensating lead wires are not connected with the cold junctions q and r of the thermocouple X, but an auxiliary thermocouple Y consisting of metals is coupled to either the cold junction q or r to provide cold junction compensation. Because the cold junctions q and r are at the same temperature, the auxiliary thermocouple Y can be connected to any of the cold junctions q and r. The auxiliary thermocouple Y acts as a means for connecting a measuring instrument to the cold junction q or r, but it develops no thermo emf. The two wires R of the auxiliary thermocouple are made from the same material. In this case, the thermo emf to be measured corresponds to the area of the portion n.sub.1 n.sub.2 n.sub.4 n.sub.3. This portion is divided into a region n.sub.5 n.sub.2 n.sub.4 n.sub.6 and a region n.sub.1 n.sub.5 n.sub.6 n.sub.3 at a temperature of T.sub.1. The thermo emf corresponding to the area of the region n.sub.5 n.sub.2 n.sub.4 n.sub.6 is directly measured by the thermocouple X. With respect to the remaining region n.sub.1 n.sub.5 n.sub.6 n.sub.3, cold junction compensation is provided by the auxiliary thermocouple Y. The temperature of the hot junction of the main thermocouple X is the sum of the temperature measured by the main thermocouple X and the temperature of the hot junction of the auxiliary thermocouple Y measured by the auxiliary thermocouple Y. The conventional thermocouples have been summarized thus far. These devices are used primarily to measure the temperatures of materials lower than 2000.degree. C.
In recent years, the industry required the measurement of higher temperatures used in heat treatment, such as, HIP and hot pressing. Presently, there is a demand for a thermocouple capable of measuring high temperatures exceeding 2000.degree. C. A tungsten/tungsten-rhenium thermocouple and other thermocouples have been made available to measure such very high temperatures. However, these conventional thermocouples are low in mechanical rigidity and so they easily break. Also, the operation suffers from instability because of thermo emf drift. Further, they have a fundamental disadvantage in that they cannot be used in a stable manner over a long period of time due to metallurgical problems. In an attempt to solve these problems, thermocouples consisting of combinations of semiconductor ceramics, such as B.sub.4 C/C (graphite), C (graphite)/ZrB.sub.2, and C (graphite)/TiC, which are stable at high temperatures were fabricated in 1950s, for enabling reliable measurement of temperatures. As a result, excellent ceramic thermocouples have now begun to be commercially available. However, various problems must still be solved to measure high temperatures using ceramic thermocouples as described below.
(1) Since ceramic thermocouples produce larger thermo emfs than the conventional metallic thermocouples, appropriate compensating lead wires for providing cold junction compensation do not exist or, if they exist, they are difficult to manufacture for technical or economical reasons. Consequently, no practically usable compensating lead wires are available.
(2) Therefore, it is necessary to water- or air-cool the cold junctions from which a thermo emf is taken by the use of a cooling equipment. Thus, the temperature of the cold junction is made uniform, and the cold junctions are temperature-compensated.
(3) Since constraints are imposed on the manufacture, long ceramic thermocouples cannot be fabricated. This forces the use of short ceramic thermocouples. Where such a short ceramic thermocouple is used while subjecting the cold junctions to forced cooling, the cooling action of water- or air-cooling reaches the hot junction, since the two elements of the thermocouple are short. The temperature of the hot junction may be rendered lower than the temperature of the material being tested. The result is that an error is introduced to the measured temperature. Also, where the cold junctions are subjected to forced cooling, a temperature difference is produced between the cold junctions because the two elements of the thermocouple differ in heat capacity and thermal conductivity. As a result, the measured temperature has an error.
In addition, ceramic thermocouples differ greatly in thermoelectromotive characteristic and, therefore, when the obtained thermal emfs are converted into temperature, compensations should be made taking the characteristics of individual ceramic thermocouples into account. However, a converter is not found which can appropriately establish translation tables used for conversion of thermo emf into temperature according to each individual ceramic thermocouple.